make keccak module configurable

[steffenix]

I am building a project using ethers that relies on ex_abi. I am having a challenge with the ExKeccak library using it with nerves. Ethers allow for a different library to be used but ex_abi uses ExKeccak, this PR is to make the keccak library used by ex_abi configurable.

[ayrat555]

thank you for the contribution. sorry for a late reply. I didn't see the pr. unfortunately github doesn't notify about new issues/prs.

for future mention contact me in telegram Ayrat55 or mention me directly in prs